The present invention relates to a magnetic random access memory (MRAM) device, and more particularly, to a spin transfer torque MRAM device including multiple magnetic free layers coupled in series.
Spin transfer torque magnetic random access memory (STT-MRAM) is a new class of non-volatile memory, which can retain the stored information when powered off. An STT-MRAM device normally comprises an array of memory cells, each of which includes at least a magnetic memory element and a selection element coupled in series between appropriate electrodes. Upon application of a switching current to the magnetic memory element, the electrical resistance of the magnetic memory element would change accordingly, thereby switching the stored logic in the respective memory cell.
FIG. 1 shows a conventional memory element for an STT-MRAM device comprising a magnetic reference layer 50 and a magnetic free layer 52 with an insulating tunnel junction layer 54 interposed therebetween, thereby collectively forming a magnetic tunneling junction (MTJ) 56. The magnetic reference layer 50 and free layer 52 have magnetization directions 58 and 60, respectively, which are substantially perpendicular to the layer planes. Therefore, the MTJ 56 is a perpendicular type comprising the magnetic layers 50 and 52 with perpendicular anisotropy. Upon application of a switching current to the perpendicular MTJ 56, the magnetization direction 60 of the magnetic free layer 52 can be switched between two directions: parallel and anti-parallel with respect to the magnetization direction 58 of the magnetic reference layer 50. The insulating tunnel junction layer 54 is normally made of a dielectric material with a thickness ranging from a few to a few tens of angstroms. When the magnetization directions 60 and 58 of the magnetic free layer 52 and reference layer 50 are substantially parallel, electrons polarized by the magnetic reference layer 50 can tunnel through the insulating tunnel junction layer 54, thereby decreasing the electrical resistivity of the perpendicular MTJ 56. Conversely, the electrical resistivity of the perpendicular MTJ 56 is high when the magnetization directions 58 and 60 of the magnetic reference layer 50 and free layer 52 are substantially anti-parallel. Accordingly, the stored logic in the magnetic memory element can be switched by changing the magnetization direction 60 of the magnetic free layer 52.
One of many advantages of STT-MRAM over other types of non-volatile memories is scalability. As the size of the perpendicular MTJ 56 is reduced, however, the thermal stability of the magnetic layers 50 and 52, which is required for long term data retention, also degrades with miniaturization of the perpendicular MTJ 56. While the thermal stability of the perpendicular MTJ 56 may be improved by increasing the anisotropies and/or the magnetic moments of the magnetic layers 50 and 52, doing so would adversely increase the current required to switch the magnetization direction 60 of the magnetic free layer 52.
For the foregoing reasons, there is a need for an STT-MRAM device that has a thermally stable perpendicular MTJ memory element which can be programmed with a low switching current.